Magnetocaloric materials, also referred to as thermomagnetic materials, can be used in magnetic cooling, in heat pumps or air conditioning systems, and generators.
Such materials are known in principle and are described, for example, in WO 2004/068512. Magnetic cooling techniques are based on the magnetocaloric effect (MCE) and may constitute an alternative to the known vapor circulation cooling methods. In a material which exhibits a magnetocaloric effect, the alignment of randomly aligned magnetic moments by an external magnetic field leads to heating of the material. This heat can be removed from the MCE material to the surrounding atmosphere by a heat transfer. When the magnetic field is then switched off or removed, the magnetic moments revert back to a random arrangement, which leads to cooling of the material below ambient temperature. This effect can be exploited for cooling purposes; see also Nature, Vol. 415, Jan. 10, 2002, pages 150 to 152. Typically, a heat transfer medium such as water is used for heat removal from the magnetocaloric material.
Customary materials are prepared by solid phase reaction of the starting elements or starting alloys for the material in a ball mill, subsequent pressing, sintering and heat treatment under inert gas atmosphere and subsequent slow cooling to room temperature. Processing by means of melt spinning is also possible. This makes possible a more homogeneous element distribution, which leads to an improved magnetocaloric effect.
A problem in the case of use of the heat exchange medium or heat transfer media is the corrosion tendency of the magnetocaloric materials. Attempts are being made in different ways to prevent this corrosion. Corrosion is also referred to as fouling or leaching.
Specifically the washout of toxic metals such as arsenic or manganese is problematic. In general, the application properties of the magnetocaloric materials suffer as a result of corrosion, fouling or leaching.
US 2007/0220901 describes providing an oxidation-resistant film on the surface of the magnetic material particles. This film is especially aluminum oxide or aluminum nitride.
JP-A-2006-124783 describes the coating of the magnetic particles with a chemical film based on phosphoric acid for corrosion protection.
JP-A-2005-226125 describes metal plating for coating of magnetic particles, the intention being that ion plating with a corrosion-resistant metal should improve the corrosion protection.
These methods envisage complex further treatment of the magnetocaloric materials.